What cycle time is and how to measure it per part

You measure a machine's real cycle time by timing the cycles it actually runs, from one cycle start to the next, over 10 to 20 consecutive cycles once the process has settled. Take the median of those times, not the mean, and divide by the parts each cycle makes to get the time per part. The programmed time is a prediction; the timed median is what the machine does.

This guide is for engineers and leads who need one cycle-time number they can defend in a quote, a schedule and an OEE review. It works one molding press through by hand, two ways.

What is cycle time?#

The Lean Enterprise Institute puts the key condition plainly: cycle time is the time to produce a part or complete a process "as timed by actual measurement".^[1] A number on a setup sheet, a CAM printout or a routing is an estimate until someone times it. The glossary entry for cycle time separates the figure you enter from the one the machine reports.

Total parts divided by total time is not a cycle time. It is a production rate for the whole period, with every stop and every slow start inside it. For example, a press that makes 3,600 parts in a 450-minute shift averages one part every 7.5 seconds, even though each part takes 6.5 seconds to mold.

Two neighboring terms get mixed up with cycle time. Takt time is the pace customer demand requires, and lead time is how long an order takes from release to delivery. A machine can cycle faster than takt and still miss demand once stops and scrap are counted, so compare takt with planned time per good part, as the flow KPIs show. Takt time, cycle time and lead time works all three through one example.

Machine, part and operator cycle time#

Three cycle times describe one station, and each answers a different question.

LEI defines machine cycle time as the time a machine requires to complete all of its operations on one piece, and operator cycle time as the time an operator takes to complete all the work elements at a station before repeating them, measured by direct observation.^[1] Part cycle time is the figure quotes and costs use.

Parts per cycle is the number of finished parts one machine cycle makes. A 4-cavity mold makes four parts per shot, and a two-station fixture on a mill makes two per cycle. Keep the cycle time per cycle and record the multiple separately, as the quantity per cycle setting does. When a cavity is blocked off, parts per cycle drops to three and the cycle time stays the same.

The operator cycle decides who sets the pace. When the operator works while the machine cuts, the station repeats at the slower of the two. On the VF-2 above, the 2 min 10 s loop fits inside the 7 min 30 s cycle, so the machine sets the pace. When the operator and machine take turns, add the two.

Programmed, actual and ideal cycle time#

Three versions of the machine cycle time exist for any operation. Each has one job, and the trouble starts when one stands in for another.

The programmed cycle time is what the part program or the machine's setpoints predict. It leaves out time the machine adds. A CAM estimate skips acceleration, tool changes and probing, and a molder's setpoints skip mold movement and the take-out robot clearing the mold.

The actual cycle time is what this guide measures. The ideal cycle time is a target you choose from the actuals on a good day, and setting the ideal cycle time covers who owns it and what a wrong one does to OEE.

You don't need the programmed time once the machine has run the job. Replace it with a timed figure after the first settled run.

How do you measure the real cycle time?#

You measure it one of three ways: with a stopwatch, from the machine's control, or from a sensor on the machine. All three time the same interval when you set them up to, and they differ in how many cycles they see.

Default to a sensor when you want every cycle on every machine, whatever its age. A stopwatch study is enough for one quote or one check, and the control adds the program name where a plant has already connected it.

With a stopwatch#

1. Pick a settled operation

   Choose a job past its first parts off, with the machine warm and the program or mold proven.

2. Choose the start point

   Pick one visible event, such as door close on a mill or mold close on a press, and time from it to the same event in the next cycle.

3. Time 10 to 20 consecutive cycles

   Write every time down, including the odd ones, and note what happened in any cycle that ran long.

4. Take the median

   Sort the times and take the middle value. The median section explains why it beats the mean.

5. Divide by parts per cycle

   Divide the median by the parts one cycle makes to get the part cycle time.

From the control or a sensor#

A CNC control or a molder's controller times its own cycles. MTConnect, the open standard many CNC controls report through, defines a process timer as the time from the beginning of production of a part on a piece of equipment until production of that part is complete.^[4] A connection to the control is a security concern for many plants, and IT usually reviews it first.

A vibration sensor on the outside of the machine times cycles from the machine's own signal. A detection algorithm decides where one cycle ends, and the right one depends on the machine type: a burst per part on a mill, an impact per stroke on a press, a steady trace on a molder. The algorithm guide matches each one to its machine. Cycle time and part count come from the same detected cycles, so part counting methods apply here too.

Traps that skew a measured cycle#

Most bad cycle times come from timing the right machine over the wrong interval. Check for these four before you trust a number.

• Averaging across stops. Dividing a shift by its part count folds every stop into the cycle. Time cycles only while the machine runs, and leave a cycle with a stop inside it out of the average.
• Load and unload in one study and out of the next. Timing cut to idle one day and door close to door close the next gives two numbers for one cycle. Time start to start on every machine and write the convention down.
• First-part-off cycles. The first cycles after a setup or mold change run slow while the operator checks the part and the mold comes to temperature. Start timing once the process has settled.
• Mixed jobs in one window. A window that spans a job change averages two operations. Time one operation at a time.

Median or mean, and why the spread matters#

Use the median of the timed cycles as the machine's cycle time. NIST's statistics handbook explains why: extreme values in the tails distort the mean, while the median, based on ranks, is not distorted by them.^[2] Cycle times are skewed in exactly this way. A cycle can run 15 seconds long when a part sticks, but it can't run 15 seconds short.

The spread tells you as much as the middle value. NIST defines the range as the largest value minus the smallest, and warns that it depends on the two most extreme points; the interquartile range covers the middle half of the data instead.^[3] Report both the median and the spread of the cycles.

• A tight spread (a few percent of the cycle) means the machine sets the pace. The median is a safe number to quote from.
• A wide spread means the operator or the material sets the pace. On an operator-loaded mill with cycles from 6 to 9 minutes, time more cycles and find out which work element varies before you quote.
• A spread that grows over days points at the process. Tool wear lengthens cycles a few seconds at a time.

Worked example: a 150-ton molding press#

For example, a 150-ton injection molding press runs a 4-cavity mold for a polypropylene cap. The setup sheet's setpoints add up to a 24-second cycle. Here is the real cycle, computed two ways.

The two methods agree within 0.2 seconds a shot. The difference is short pauses, too brief to count as stops, that sit inside the running time and outside the timed shots.

The wrong method reads 27,000 ÷ 900 = 30.0 seconds a shot, 15.4% longer than the press really cycles. That number belongs to availability, not to the cycle. The programmed 24 seconds is wrong in the other direction: a quote built on it prices every part 0.5 seconds too fast.

Measure cycle times automatically#

IoTFlows puts a SenseAi on each machine. It reads vibration and sound from outside, with no PLC connection, and reports part counts and cycle times derived from the machine's cycle.

The actual cycle time IoTFlows reports is the running time between counted parts, so stops stay out of it, as the metrics reference defines. Each operation carries the detection algorithm for its machine type: Discrete Analysis on CNC mills, Discrete w/o Merge on presses, and Continuous Analysis on molders, which divides running time by the shot cycle you timed. Shift Production shows the ideal, goal and actual cycle side by side, and production monitoring puts every count against the shift goal.

Jesel, a valvetrain manufacturer, uses this to compare "actual measured cycle times against the times specified in the bills of material", as its customer story describes.

Which cycle time should you use?#

Use a different cycle time for each job, and never let one stand in for another.

A quote also needs the setup share per part. LEI's effective machine cycle time adds load and unload and the changeover spread over the batch to the machine cycle.^[1]

For schedules, plan on real run rates, which fold in the stops and scrap a quote leaves out.